The present invention is generally directed to toner compositions, and processes thereof. More specifically, the present invention is directed to toner compositions comprised of a dispersion of a wax in toner resins, such as polyesters, especially polyesters that can be selected for the preparation of heat fixable toners with, for example, excellent low temperature fixing characteristics and superior offset properties in a hot roll fixing system, and with excellent vinyl offset properties. Dispersion of waxes in, for example, toner resins like polyesters is difficult, and cannot be effectively accomplished because of the incompatibility of the wax and the polyester resulting in large wax domains. When large wax domains are present, upon micronization of the toner thereof, there is a propensity to cause the formation of toner particles containing primarily the wax, and particles composed primarily, or exclusively of the wax. The aforementioned particles with high wax content will not, for example, charge the same as the remainder of the toner components. The free wax particles can eventually collect on a donor roll or photoreceptor and cause the filming thereof. It is, therefore, advantageous that these wax particles be retained in the toner. Also, the toner pigments are not easily dispersed in the wax causing both development and fuser ghosting problems. The toner and processes of the present invention enable, for example, the effective consistent roll fusing of the resulting toners with polyester resins, and there can be selected low amounts of fuser oil, reference for example U.S. Pat. No. 5,124,224, the disclosure of which is totally incorporated herein by reference. In embodiments, the present invention is directed to a toner comprised of resin, pigment and wax, particularly a low molecular weight wax with a molecular weight of from about 1,000 to about 20,000 and ethoxylated long chain alcohols which are believed to be block copolymers with surfactant properties, and are available from Petrolite Specialties Polymers Group, Tulsa, Okla. as, for example, UNITHOX 420.RTM., 450.RTM., 480200 , 520.RTM., 550.RTM., 720.RTM. and 750.RTM..
Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black is utilized often, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and negative charge enhancing additives, such as zinc and aluminum salts of tertiary butyl salicylic acid, and the like, can be selected.
Toner can be fixed to a support medium, such as a sheet of paper or transparency, by different fixing methods. A fixing system which is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes is hot roll fixing. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is referred to as the offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs as determined by, for example, a creasing or an abrasion test. The difference between MFT and HOT is known as the fusing latitude.
The hot roll fixing system described herein and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature enabling fixing to a support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Further, offsetting can be a problem. Also, toners containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface. Toners prepared from certain polyesters generally exhibit little or no vinyl offset.
There is a need for a toner which has low fix temperature and high offset temperature (or wide fusing latitude), and superior vinyl offset property, and processes for the preparation of such a toner. Toners which operate at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toners, that is toners with a MFT lower than 200.degree. C., and preferably lower than 160.degree. C., would reduce the volatilization of release oil such as silicone oil which may occur during high temperature operation and which can cause problems when the volatilized oil condenses in other areas of the copying machine. In particular, toners with wide fusing latitude and with acceptable toner particle elasticity are needed. Toners with wide fusing latitude can provide flexibility in the amount of oil needed as release agent and can minimize copy quality deterioration related to the toner offsetting to the fuser roll.
To lower the minimum fix temperature of the binder resin, in some instances the molecular weight of the resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used for low temperature fixing toners. For example, polyester resins as a binder for toner are disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
Another method of improving offset resistance is to utilize a crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,941,898 to Sadamatsu et al. discloses a toner in which a crosslinked vinyl type polymer is used as the binder resin. Similar disclosures for vinyl type resins are made in U.S. Pat. Re. Nos. 31,072 (a reissue of 3,938,992) to Jadwin et al., 4,556,624 to Gruber et al., 4,604,338 to Gruber et al. and 4,824,750 to Mahalek et al.
While significant improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue in that with crosslinked resins prepared by conventional polymerization, that is crosslinking during polymerization using a crosslinking agent, there exist three types of polymer configurations: a linear and soluble portion referred to as the linear portion, a portion comprising highly crosslinked gel particles, which is not soluble in substantially any solvent, for example tetrahydrofuran, toluene and the like, and is referred to as a gel, and a crosslinked portion, which is low in crosslinking density and, therefore, is soluble in some solvents like tetrahydrofuran, toluene and the like, and is referred to as the sol. The presence of highly crosslinked gel in the binder resin increases the hot offset temperature, but at the same time the low crosslink density portion or sol increases the minimum fix temperature. An increase in the amount of crosslinking in these types of resins results in an increase not only of the gel content, but also an increase in the amount of sol or soluble crosslinked polymer with low degree of crosslinking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. Also, a drawback of certain crosslinked polymers prepared by conventional polymerization is that as the degree of crosslinking increases, the gel particles or very highly crosslinked insoluble polymer with high molecular weight grow larger. The large gel particles can be more difficult to disperse pigment in, causing the formation of unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, compatibility with other binder resins may be relatively poor and toners containing vinyl polymers often show vinyl offset.
Crosslinked polyester binder resins prepared by conventional polycondensation reactions have been prepared for improving offset resistance, such as disclosed, for example, in U.S. Pat. No. 3,681,106 to Burns et al. As with crosslinked vinyl resins, increased crosslinking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When crosslinking is carried out during polycondensation using tri- or polyfunctional monomers as crosslinking agents with the polycondensation monomers, the net effect is that apart from generating highly crosslinked high molecular weight gel particles, which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part branches due to the formation of sol or crosslinked polymer with a very low degree of crosslinking, which is soluble in some solvents. These intermediate high molecular weight species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase. Furthermore, gel particles formed in the polycondensation reaction, which is carried out using conventional polycondensation in a reactor with low shear mixing, can grow rapidly with increase in degree of crosslinking. As in the case of crosslinked vinyl polymers using conventional polymerization reactions, these large gel particles may be more difficult to disperse pigment in resulting in unpigmented toner particles after pulverization, and thus hindering developability. Another consequence of incorporating branching components is a decrease in micronization rates from 1.2 to twice that of the linear polymer.
U.S. Pat. No. 4,533,614 to Fukumoto et al. discloses a crosslinked polyester binder resin which evidences low temperature fix and good offset resistance. Similar disclosures are presented in U.S. Pat. No. 3,681,106 and Japanese Laid-Open Patent Applications 943,62/1981, 1160,41/1981 and 166,651/1980. As discussed in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, with colored toners other than black, metal complexes can adversely affect the color of pigments. It is also known that metal containing toner can have disposal problems in some geographical areas, such as for example in the state of California, U.S.A. Metal complexes are often also expensive materials.
Also, to prevent fuser roll offsetting and to increase fuser latitude of toners, the toner composition can be modified. For example, waxes, such as low molecular weight polyethylene, polypropylene, and the like, have been added to toners to increase their release properties, as disclosed in U.S. Pat. No. 4,513,074 to Nash et al., the disclosure of which is totally incorporated herein by reference. However, to prevent offset and insure the functionality of the toner the wax selected should be very well dispersed with the average volume diameter of the dispersed wax phase preferably ranging from about 0.1 micron to about 2 microns. It is known that achievement of this level of wax dispersion is difficult to accomplish in low shear compounding equipment, such as extruders and with certain functional materials like certain polyesters, or with low molecular weight resins. Another difficulty which arises is mismatch of viscosity among toner components, for example because certain polyesters suitable for roll fusing are of sufficiently high molecular weight to prevent offset their melt viscosity is significantly higher than that of the wax at the melt mixing temperature.
The problem associated with the dispersion of waxes, especially polyethylene waxes like POLYWAX.RTM., obtained from Petrolite Corporation, in low melt viscosity polyesters can be eliminated or minimized with the toners and processes of of the present invention.